The polymerization of styrene is a very important industrial process that supplies materials used to create a wide variety of polystyrene-containing articles. This expansive use of polystyrene results from the ability to control the polymerization process. Thus, variations in the polymerization process conditions are of utmost importance since they in turn allow control over the physical properties of the resulting polymer. The resulting physical properties determine the suitability of polystyrene for a particular use. For a given product, several physical characteristics must be balanced to achieve a suitable polystyrene material. Among the properties that must be controlled and balanced are average molecular weights (Mn, Mw and Mz) of the polymer, molecular weight distribution (MWD), melt flow index (MFI), and the glass transition temperature (Tg).
U.S. Pat No. 5,540,813 by Sosa, et. al., which is incorporated herein by reference, discloses a process for preparing monovinyl aromatic polymers, such as polystyrene, which utilizes a combination of sequentially ordered multiple reactors, heat exchangers and devolatilizers to strictly control polymer properties such as the molecular weight distribution and melt flow index.
The relationship between the molecular weight and the storage modulus is of importance in polymer foam applications. Such foam applications require high molecular weight polymers having a high storage modulus. It is thought that the storage modulus is related to the degree of branching along the polymer chain. As the degree of branching increases, the likelihood that a branch entangles with other polymer chains increases. A polymer product having a higher degree of branching or cross-linking tends to have a higher storage modulus and, therefore, better foam stability characteristics.
Methods for preparing branched polymers are well-known in the art. For example, the preparation of branched polystyrene by free radical polymerization has been reported. However, this method increases the branching in the devolatilization step and produces a polymer with an undesirably low molecular weight.
Rather than employing free radical polymerization, some have used multi-functional mercaptans to form branched polymers. While materials having an acceptable molecular weight can be prepared by this method, these products are unacceptable for foam applications due to their undesirable flow properties.
The properties of randomly branched polystyrene prepared in the presence of divinylbenzene have been reported by Rubens (L. C. Rubens, Journal of Cellular Physics, pp 311-320, 1965). However, polymers having a useful combination of molecular weight and cross-linking are not attainable. At low concentrations of divinylbenzene, low molecular weight polymers having little branching result. However, higher concentrations of the cross-linking agent result in excessive cross-linking and concomitant gel formation that is highly undesirable in industrial polystyrene processes. Similar results and problems were reported by Ferri and Lomellini (J. Rheol. 43(6), 1999).
Commercial polystyrene made by the conventional free-radical process yields linear structures. As noted, methods to prepare branched polystyrenes, however, are not easily optimized and few commercial non-linear polystyrenes are known. Studies of branched polymers show that these polymers possess unique molecular weight-viscosity relationships due to the potential for increased molecular entanglements. Depending upon the number and length of the branches, non-linear structures can give melt strengths equivalent to that of linear polymers at slightly higher melt flows.
U.S. Pat. No. 6,353,066 to Sosa describes a method of producing a copolymer by placing a vinylbenzene (e.g. styrene) in a reactor, placing a cross-linking agent (e.g. divinylbenzene) in the reactor, and placing a chain transfer agent (e.g. mercaptan) in the reactor and forming a polyvinylbenzene in the presence of the cross-linking agent and chain transfer agent.
It would be desirable if methods could be devised or discovered to provide vinylaromatic polymers with increased branching, such as branched polystyrene with improved properties. It would also be helpful if a method could be devised that would help optimize the physical properties of vinylaromatic polymers having increased branching. Such polymers may have a higher melt strength than polymers with linear chains, and may improve processability and mechanical properties of the final product (e.g. increase density in foam application).